With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Under these circumstances, the lithium ion secondary batteries having advantages such as a high charge/discharge voltage and a large charge/discharge capacity have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure, LiMnO2 having a zigzag layer structure, LiCoO2, LiCo1-xNixO2 and LiNiO2 having a layer rock-salt structure, or the like. Among the secondary batteries using these active substances, lithium ion secondary batteries using LiCoO2 are excellent in view of a high charge/discharge voltage and a large charge/discharge capacity thereof. However, owing to use of the expensive Co, various other positive electrode active substances have been studied as alternative substances of LiCoO2.
On the other hand, lithium ion secondary batteries using LiNiO2 have also been noticed because they have a high charge/discharge capacity. However, since the material LiNiO2 tends to be deteriorated in thermal stability and durability upon charging and discharging, further improvements on properties thereof have been demanded.
Specifically, when lithium is released from LiNiO2, the crystal structure of LiNiO2 suffers from Jahn-Teller distortion since Ni3+ is converted into Ni4+. When the amount of Li released reaches 0.45, the crystal structure of such a lithium-released region of LiNiO2 is transformed from hexagonal system into monoclinic system, and a further release of lithium therefrom causes transformation of the crystal structure from monoclinic system into hexagonal system. Therefore, when the charge/discharge reaction is repeated, the crystal structure of LiNiO2 tends to become unstable, so that the resulting secondary battery tends to be deteriorated in cycle characteristics or suffer from occurrence of undesired reaction between LiNiO2 and an electrolyte solution owing to release of oxygen therefrom, resulting in deterioration in thermal stability and storage characteristics of the battery. To solve these problems, various studies have been made on the LiNiO2 materials to which Co, Al, Mn, Ti, etc., are added by substituting a part of Ni in LiNiO2 therewith.
That is, by substituting a part of Ni in LiNiO2 with different kinds of elements, it is possible to impart properties inherent to the respective substituting elements to the LiNiO2. For example, in the case where a part of Ni in LiNiO2 is substituted with Co, it is expected that the thus substituted LiNiO2 exhibits a high charge/discharge voltage and a large charge/discharge capacity even when the amount of Co substituted is small. On the other hand, LiMn2O4 forms a stable system as compared to LiNiO2 or LiCoO2, but has a different crystal structure, so that the amounts of the substituting elements introduced thereto are limited.
In consequence, in order to obtain Co- or Mn-substituted LiNiO2 having a high packing property and a stable crystal structure, it is required to use a nickel-cobalt-manganese-based precursor which is well controlled in composition, properties, crystallizability and particle size distribution.
In particular, the particle size distribution of the positive electrode active substance for non-aqueous secondary batteries such as LiNiO2 has a large contribution to a packing property of a positive electrode material. Therefore, there is a strong demand for positive electrode active substances having a more uniform particle size distribution. For this reason, nickel-cobalt-manganese-based compound particles as a precursor of the LiNiO2 substituted with different kinds of elements have also been required to have a uniform particle size and a less content of very fine particles.
It is conventionally known that nickel-cobalt-manganese-based compound particles are controlled in tap density, particle shape and particle size distribution (Patent Documents 1 to 4).
The technique describe in Patent Document 1 relates to spherical high-density cobalt/manganese co-precipitated nickel hydroxide having a tap density of not less than 1.5 g/cc.
In addition, in Patent Document 2, there is described a nickel/manganese co-precipitated composite oxide in which a transition metal element is uniformly incorporated in an atomic level in the form of a solid solution.
Further, in Patent Document 3, there is described a nickel/cobalt/manganese composite oxyhydroxide which is synthesized by reacting nickel/cobalt/manganese co-precipitated composite oxide aggregated particles with an oxidizing agent.
Furthermore, in Patent Document 4, there is described a nickel/cobalt/manganese composite hydroxide which is controlled in particle size distribution.